//全局注册自定义指令，用于判断当前图片是否能够加载成功，可以加载成功则赋值为img的src属性，否则使用默认图片
export let realImgCallback = async function (el, binding) {//指令名称为：real-img
  let imgURL = binding.value;//获取图片地址
  if (imgURL) {
    let exist = await imageIsExist(imgURL);
    if (exist) {
      el.setAttribute('src', imgURL);
    }
  }
};

/**
 * 检测图片是否存在
 * @param url
 */
let imageIsExist = function (url) {
  return new Promise((resolve) => {
    var img = new Image();
    img.onload = function () {
      if (this.complete == true) {
        resolve(true);
        img = null;
      }
    };
    img.onerror = function () {
      resolve(false);
      img = null;
    };
    img.src = url;
  })
};
